System and method for intrusion detection using a time domain radar array

ABSTRACT

A system and method for highly selective intrusion detection using a sparse array of time modulated ultra wideband (TM-UWB) radars. Two or more TM-UWB radars are arranged in a sparse array around the perimeter of a building. Each TM-UWB radar transmits ultra wideband pulses that illuminate the building and the surrounding area. Signal return data is processed to determine, among other things, whether an alarm condition has been triggered. High resolution radar images are formed that give an accurate picture of the inside of the building and the surrounding area. This image is used to detect motion in a highly selective manner and to track moving objects within the building and the surrounding area. Motion can be distinguished based on criteria appropriate to the environment in which the intrusion detection system operates.

RELATED APPLICATIONS

[0001] This application is related to U.S. Pat. application Ser. No.______ (Attorney Docket No. 1659.0670000), filed the same day as thisapplication, Jun. 14, 1999, entitled “Wide Area Time Domain RadarArray”, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to radar motiondetection, and more particularly to using a sparse array of timemodulated ultra wideband radars for highly selective intrusiondetection.

[0004] 2. Related Art

[0005] Today, many homes and businesses employ surveillance systems forintrusion detection. Consumers have spent billions of dollars on homesecurity systems over the last few years, and the number of homes withsecurity systems has increased by almost half. These systems varydramatically in sophistication and cost, but most include perimetersensors on outside doors and windows, motion detectors in key insideareas, a control unit to interpret and respond to signals from thesensors, and a siren or other alert mechanism. Most are connected to acentral monitoring station, which can notify the police in the eventsomething triggers one of the sensors.

[0006] Conventional intrusion detection systems, particularly those inthe cost range of the average home or small business owner, suffer fromvery high false alarm rates, often 90% and above. This imposesprohibitive costs on local police departments having to answer thesefalse alarms. Many cities have responded by charging fines for answeringthese calls. This in turn provides incentive to home and business ownersto deactivate the alarm system to avoid the false alarms. One studysuggests that in burglarized homes with alarm systems, almost half ofthe alarms weren't even activated.

[0007] Conventional intrusion detection systems suffer a high rate offalse alarms for many reasons. One reason is that these systems provideminimal selectivity. As used herein, selectivity refers to an intrusiondetection system's ability to distinguish movement on some basis, suchas where the movement is occurring, how fast an object is moving, or thepath that an object is moving along. Obviously, detection systems thatare more selective will likely suffer fewer false alarms becausethreatening movement can be more precisely defined and distinguishedfrom movement defined as benign. What is defined as threatening andbenign will vary by the particular environment in which the systemoperates. For instance, in a home environment, threatening movementcould be defined as movement around the outside perimeter of the house,while movement inside the house is defined as benign. Therefore, anintruder approaching a door or window from the outside would trigger thealarm, whereas a child opening a bedroom door would not.

[0008] A need therefore exists for a highly selective intrusiondetection system and method.

Summary of the Invention

[0009] Briefly stated, the present invention is directed to a system andmethod for highly selective intrusion detection using a sparse array oftime modulated ultra wideband (TM-UWB) radars. TM-UWB radars emit veryshort RF pulses of low duty cycle approaching Gaussian monocycle pulseswith a tightly controlled pulse-to-pulse interval. Two or more of theseTM-UWB radars are arranged in a sparse array (i.e., they are spaced atintervals of greater than one quarter wavelength), preferably around theperimeter of a building. Each TM-UWB radar transmits ultra widebandpulses that illuminate the building and the surrounding area. One ormore of the radars receives signal returns, and the signal return datais processed to determine, among other things, whether an alarmcondition has been triggered.

[0010] An advantage of the current invention is that ultra wideband(UWB) pulses are used. As used herein, UWB refers to very short RFpulses of low duty cycle ideally approaching a Gaussian Monocycle.Typically these pulses have a relative bandwidth (i.e., signalbandwidth/center frequency) which is greater than 25%. The ultrawideband nature of these pulses improves both angle and rangeresolution, which results in improved performance (e.g., greaterselectivity, more sensitive motion detection). The term “wavelength”, asused herein in conjunction with ultra wideband systems, refers to thewavelength corresponding to the center frequency of the ultra widebandpulse.

[0011] Another advantage of the current invention is that highresolution radar images are formed which give an accurate picture of theinside of the building and the surrounding area. The current inventionuses this image to, among other things, detect motion in a highlyselective manner and to track moving objects within the building and thesurrounding area. High resolution radar images are possible because theTM-UWB radars positioned around the perimeter of the building form asparse array capable of achieving high angular resolution. Angularresolution is a function of the width of the TM-UWB radar array, i.e.,the wider the array, the greater the angular resolution. Conventionalnarrowband radars arranged in a sparse array suffer off-axisambiguities, and are therefore not practical. However, the UWB pulsestransmitted by the TM-UWB radars are sufficiently short in duration(with very few sidelobes) that the radars can be used in a sparse arrayconfiguration without off-axis ambiguities. Furthermore, rangeambiguities are cured by time-encoding the sequence of transmittedTM-UWB pulses.

[0012] Another advantage of the current invention is that highlyselective motion detection is possible. Using the high resolution radarimages generated by the TM-UWB radar array, motion can be distinguishedbased on criteria appropriate to the environment in which the intrusiondetection system operates. For example, home security systems accordingto the present invention can distinguish outside movement around doorsand windows from movement inside the house. Alternatively, businesssecurity systems can distinguish movement in an unsecured portion of thebuilding from movement in a secured portion. This selectivity can resultin lower false alarm rates.

[0013] Another advantage of the current invention is that high angularresolution may be achieved at a low center frequency. Because thetransmitted UWB pulses have a large relative bandwidth, and because theradar array is wide, a lower center frequency can be maintained andstill achieve a high angular resolution. Operating at a lower centerfrequency relaxes the timing requirements of the system, which makes iteasier to achieve synchronization between the radars, and results inless complex, less expensive implementations. A low center frequencyalso results in UWB pulses that are able to better penetrate lossymaterials and withstand weather effects.

[0014] Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements. The drawingin which an element first appears is indicated by the leftmost digit inthe corresponding reference number.

BRIEF DESCRIPTION OF THE FIGURES

[0015] The present invention will be described with reference to theaccompanying drawings, wherein:

[0016]FIG. 1 illustrates an example building environment within whichthe present invention can be used;

[0017]FIG. 2 depicts an intrusion detection system;

[0018]FIG. 3 is a flowchart that describes the operation of theintrusion detection system;

[0019]FIG. 4 is a flowchart that describes the generation of radarimages;

[0020]FIG. 5 depicts the intrusion detection system operating in a firstmode including back scattering at each sensor and forward scattering;

[0021]FIG. 6 depicts the intrusion detection system operating in asecond mode including back scattering at one sensor and forwardscattering;

[0022]FIG. 7 depicts the intrusion detection system operating in a thirdmode including back scattering only;

[0023]FIG. 8 depicts an imaging area within an example buildingenvironment;

[0024]FIG. 9 is a flowchart that describes the generation of a radarimage;

[0025]FIG. 10 depicts example reflectograms for four sensors;

[0026]FIG. 11 is a flowchart that describes processing the radar imagesto determine whether an alarm condition has been triggered;

[0027]FIG. 12A depicts an example clutter map;

[0028]FIG. 12B depicts an example radar image with a moving target;

[0029]FIG. 12C depicts an example differential map, calculated as thedifference between the clutter map of FIG. 12A and the radar image ofFIG. 12B; and

[0030]FIG. 13 depicts a preferred calibration of the home intrusionsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Overview of the Invention

[0032] The present invention is directed to a system and method forhighly selective intrusion detection using a sparse array of TM-UWBradars. TM-UWB (or impulse) radio and radar technology was first fullydescribed in a series of patents, including U.S. Pat. Nos. 4,641,317(issued Feb. 3, 1987), 4,743,906 (issued May 10, 1988), 4,813,057(issued Mar. 14, 1989), 4,979,186 (issued Dec. 18, 1990) and 5,363,108(issued Nov. 8, 1994) to Larry W. Fullerton. A second generation ofTM-UWB patents include U.S. Pat. Nos. 5,677,927 (issued Oct. 14, 1997),5,687,169 (issued Nov. 11, 1997) and 5,832,035 (issued Nov. 3,1998) toFullerton et. al. These patent documents are incorporated herein byreference.

[0033]FIG. 1 illustrates a building environment 100 within which thepresent invention is used. The present invention includes two or moresensors 102. In a preferred embodiment, four sensors 102(102A, 102B,102C, and 102D, as shown in FIG. 1) are located around the perimeter ofa building. Using more than four sensors 102 will further reduce thefalse alarm rate. The sensors 102 communicate with each other via acommunication pathway 104. Though only a single communication pathway104 is shown, each sensor 102 can communicate with one or more of theother sensors 102.

[0034] The example building depicted in FIG. 1 includes perimeter(outside) walls 106, inside walls 112, doors 110, and windows 108. Theareas in and around the building are conveniently divided into inside114 and outside 116. Those skilled in the art will recognize that thebuilding shown in FIG. 1 is only a simple example, and that the conceptsdescribed herein apply equally well to any arbitrarily shaped building,with any configuration of doors, windows, interior walls, andfurnishings.

[0035] One of the primary objects of the present invention is to detectmovement of objects in and around a perimeter, such as outside walls ofa building. A perimeter may alternatively be defined as two boundariesto allow for noise and clutter variations. In a two boundary system, theperimeter may be defined as an inside and outside boundary separated bysome distance (e.g. 2 ft). An object on the outside would have to crossthe inside boundary to trigger an entry alarm; whereas, an object on theinside would have to cross the outside boundary to trigger an exitalarm.

[0036] The present invention will be described in an example embodimentwhere movement of object are detected in and around the building shownin FIG. 1. For convenience, both an inside target 118 and an outsidetarget 120 are shown. The following discussion will refer to bothcollectively as targets.

[0037]FIG. 2 depicts the components of the present invention in greaterdetail, referred to collectively as an intrusion detection system 200.Each sensor 102 preferably includes a TM-UWB radar 202, and a wirelesslink 204. The sensors 102 communicate with a processor 206 that isresponsible for processing the data received by the sensors anddetermining whether an alarm condition has been met. Note that, forpurposes of clarity, only two sensors 102 (A and B) are depicted in FIG.2. As stated above, intrusion detection system 200 includes two or moresensors 102.

[0038] TM-UWB radar 202 is preferably implemented as described in U.S.Pat. Nos. 4,743,906, and 5,363,108, incorporated by reference above.However, those skilled in the art will recognize that the conceptsdescribed herein apply equally well to other radars that transmit timemodulated UWB pulses.

[0039] TM-UWB radars 202 transmit UWB pulses and at least one receivessignal returns, depending on the particular mode of operation (describedbelow). Each TM-UWB radar 202 can utilize a single antenna element 208for both transmission and reception, separate antenna elements fortransmission and reception, or even an array of antenna elements fortransmission and reception, including phased arrays of antennas. Thoseskilled in the art will recognize that the number and type of antennaelements will vary based on the particular application and desiredtransmission characteristics.

[0040] TM-UWB radar 202 preferably operates with a center frequencybetween 1 GHz and 3 GHz, and a pulse repetition rate of 1.25 MHZ. Othercenter frequencies are possible, though hydrometer effects introduceproblems around 10 GHz and above. Similarly, the pulse repetition ratewill vary based on the particular embodiment. Note that if the timemodulation of the UWB pulses includes a random component, pseudo-randomnoise (rather than true noise) is used so that the noise sequence can bereproduced at the other radars. A good discussion of time modulationusing pseudo-random noise codes for impulse systems is found in U.S.Pat. No. 5,677,927 (hereafter the '927 patent), incorporated byreference above.

[0041] Sensors 102 placed along the perimeter of a building will clearlybe separated by more than a quarter wavelength at these centerfrequencies. The sensors therefore form a sparse array. Sparse arrays ofTM-UWB radars are discussed in detail in commonly owned, co-pending U.S.Pat. application Ser. No. ______ (attorney docket 1659.0670000), filedthe same day of the present application, Jun. 14, 1999, entitled “WideArea Time Domain Radar Array,” which has been incorporated by reference.Sensors 102 are preferably packaged for convenient installation in aconventional wall electrical socket, securely fastened such that itcannot easily be removed. Those skilled in the art will recognize thatthree-dimensional images may be obtained by ensuring that all thesensors 102 do not occupy the same horizontal plane, i.e., at least onesensor 102 occupies a horizontal plane different from the other sensors102.

[0042] Processor 206 can be implemented using many differentconfigurations of computer hardware and software, as is well known tothose skilled in the art. Each particular application will dictate theprocessing needs of the system, size requirements, memory requirements,and other implementational details. Processor 206 can be physicallylocated at any convenient location. Processor 206 can be included in thesame packaging with a sensor 102, or close enough to a sensor such thatdata may be transferred between processor 206 and the nearby sensor viaa cable. Alternatively, processor 206 can be physically distant from allsensor 102 and communicate with one or more of them wirelessly.

[0043] Communication pathway 104 represents a wire or wirelesstransmission medium. In a preferred embodiment, sensors 102 communicatewith each other via a wireless link, wherein communication pathway 104represents electromagnetic waves propagating through the environment.Alternatively, communication pathway 104 can be implemented as a cable(e.g., coaxial cable, optical fibre) connecting the radars.

[0044] Wireless links 204 provide for wireless communication betweensensors 102 via communication pathway 104. Wireless links can beimplemented as any number of conventional devices known to those skilledin the art, depending upon the bandwidth required by the particularapplication. However, wireless link 204 is preferably implemented as aTM-UWB radio, as described in many of the above cited patents andapplications. In this preferred embodiment, data transfers areaccomplished using subcarrier modulation as described in the '927patent, incorporated by reference above. Alternatively, a single TM-UWBradar can be configured to perform the functions of wireless link 204and TM-UWB radar 202. In other words, a single TM-UWB radar is used ateach sensor 102 to transmit UWB radar pulses and communicate wirelesslywith other sensors 102. Combining these functions into a single unitresults in less expensive implementations. Further, in modes thatinclude forward scattering, synchronization between the radars isachieved without requiring a separate synchronization signal. Note thatwireless links 204 are unnecessary for those embodiments employing acable as communication pathway 104.

[0045] Wireless links 204 are responsible for, inter alia, transmittingscattering data received by their associated radars 202, and exchangingsynchronization information when forward scattering data is being taken.The bandwidth requirements for wireless links 204 depend upon the typesof data analysis performed by processor 206, the rate at which TM-UWBradar 202 transmits UWB pulses and various other factors. Wireless links204 can also be either bidirectional or simplex, depending upon therequirements of the application. Those skilled in the art will recognizethe cost to benefit tradeoff associated with conventional wirelessimplementations. Other implementations are discussed below.

[0046] Operation of the Current Invention

[0047]FIG. 3 is a flowchart that describes the operation of the currentinvention. This section provides an overview of the operation. Each stepis then described in detail in the following sections.

[0048] In step 302, intrusion detection system 200 is calibrated.Calibration as used herein refers to, among other things, identifyingthe positions of the various sensors 102 and one or more security zones.A security zone, as described below, is an area in which certainmovement could trigger an alarm condition. The calibration of step 302is performed before intrusion detection system 200 begins monitoringbuilding environment 100. Further details regarding calibration areprovided after detailed discussions of the next two steps.

[0049] In step 304, a radar image is generated by the operation ofintrusion detection system 200. The sensors 102 transmit UWB pulses,preferably in a omnidirectional manner, and then receive the reflectedenergy, referred to herein as signal returns or signal return data.Processor 206 then creates a radar image based on the signal return datacollected by all sensors 102.

[0050] In step 306, processor 206 determines whether an alarm conditionhas been met. This determination is based on the current radar image,and in many cases, on past radar images as well. Intrusion detectionsystem 200 triggers various alarms in the event that an alarm conditionis met, such as lights, sirens, and calls to emergency personnel.

[0051] The following sections described each step in detail.

[0052] Generation of Radar Images

[0053]FIG. 4 is a flowchart that describes step 304 in greater detail.In step 402, flow proceeds to step 404 only for those embodiments thatinclude forward scattering measurements. In step 404, radars 202 aresynchronized, as described in detail below. Skilled artisans willrecognize that this synchronization allows for useful analysis of thescattering data.

[0054] In step 406, each of the radars 202 transmits UWB pulses,preferably in an omnidirectional fashion, radiating the pulsed energy inall directions.

[0055] In step 408, signal returns are received by at least one radar202, depending upon the mode of operation. Intrusion detection system200 preferably operates in three different modes of operation. In allthree modes, each TM-UWB radar 202 transmits UWB pulses. The differentmodes vary based on which radars 202 are configured to receive signalreturns, and whether the radars are synchronized for forward scatteringmeasurements.

[0056]FIG. 5 depicts intrusion detection system 200 operating in a firstmode. Again, for purposes of clarity, only two sensors are depicted(102A and 102B) and a reflective body 502. Reflective body 502represents any object, either inside 114 or outside 116, that reflects aportion of the transmitted pulse energy. As shown, both TM-UWB radars202 transmit UWB pulses and receive the corresponding signal returnsreflecting off reflective body 502. This process is known to thoseskilled in the art as back scattering, or mono-static operation. Theback scattering data from each radar 202 is passed to processor 206 (notshown in FIG. 3) for analysis. As mentioned above, processor 206 can belocated in close physical proximity or connected wirelessly to any oneor more of sensors 102.

[0057] Sensors 102 also perform forward scattering (or bi-static)measurements, which refers to a TM-UWB radar 202 receiving signalreturns corresponding to UWB pulses transmitted by another sensor 102.As shown in FIG. 5, radar 202A receives signal returns corresponding toUWB pulses transmitted by radar 202B. Radar 202B passes both back andforward scattering data on to processor 206. TM-UWB radars 202 must besynchronized in order to utilize the forward scattering data. Thissynchronization is preferably implemented across communication pathway104.

[0058] Synchronizing radars 202 can be accomplished in at least twodifferent ways. In a first embodiment, a synchronization signal istransmitted between radars 202 via wireless links 204. In thisembodiment, wireless links 204 are chosen which have high temporalresolution, on the order of ten picoseconds. This resolution isnecessary to achieve the desired synchronization.

[0059] In a second embodiment, each radar 202 receives UWB pulsestransmitted by the radar 202B via two paths. As described above, radar202A receives forward scattering signal returns that reflect offreflective body 502. However, radar 202A can also receive UWB pulsesthat travel directly from radar 202B to radar 202A. These UWB pulses canbe used by radar 202A for synchronization, so long as the distancebetween the radars is known. Those skilled in the art will recognizethat the antenna 208B associated with radar 202B must be chosen suchthat its beam pattern provides for sufficient transmission in thedirection of radar 202A.

[0060]FIG. 6 depicts intrusion detection system 200 operating in asecond mode. In this mode, certain of the radars 202 are used forforward scattering purposes only, i.e., they transmit UWB pulses whichare received by other radars 202, but do not themselves receive anysignal returns. For example, in FIG. 6, radar 202B transmits UWB pulsesthat are received by radar 202A, as indicated by the forward scatteringpropagation path. Radar 202A receives the forward scattering signalreturns corresponding to UWB pulses transmitted by radar 202B, and alsoreceives its own back scattering signal returns. If intrusion detectionsystem 200 operates only in the second mode, radar 202B can beimplemented in a more simple, inexpensive manner because it need onlytransmit, not receive.

[0061] Again, the radars must be synchronized, preferably acrosscommunication pathway 104, in order to utilize the forward scatteringdata. Note that in this mode, only the radar that receives signalreturns passes data (both back and forward scattering data) to processor206 (not shown in FIG. 6) for analysis. Furthermore, communication onlyneeds to proceed in one direction between wireless links 204, i.e., fromradar 202A to radar 202B. Therefore, for embodiments only operating inthe second mode, wireless link 204B can be implemented as a receiveronly.

[0062]FIG. 7 depicts intrusion detection system 200 operating in thethird mode. In this mode, all of the radars 202 collect back scatteringdata only. As shown in FIG. 7, each radar 202 transmits UWB pulses andreceives the corresponding signal returns. The back scattering datacollected by each radar 202 is passed on to processor 206 (not shown inFIG. 7) for analysis. Note that in this mode, there is no requirementthat the radars 202 be synchronized because forward scattering data isnot being collected.

[0063] Returning to the flowchart of FIG. 4, in step 410, processor 206generates a radar image based on the signal return data collected bysensors 102. FIG. 8 depicts building environment 100 for purposes ofillustrating the analysis of back scattering data (and forwardscattering, where available) to generate an image of inside target 118.FIG. 8 also depicts an imaging area 802 that defines an example area tobe imaged. Imaging area 802 could, for example, represent a portion ofthe building inside 114, the entire inside 114, or the inside 114 andoutside 116. The needs of each particular intrusion system willdetermine which areas require surveillance, i.e., radar imaging.

[0064] A grid 804 criss-crosses imaging area 802, defining one or morevoxels 806 (a voxel is a minimum resolution portion of a threedimensional space, comparable to a pixel in two dimensional space). Asdescribed below, processor 206 calculates a value for each voxel 806indicative of the reflected energy measured in the portion of imagingarea 802 defined by that voxel. The resulting grid 804 of voxels 806forms a radar image of imaging area 802. Grid 804 is maintained inprocessor 206, and can vary in spacing to define voxels 806 havingdifferent resolution (grid 804 need not be orthogonal). Decreasing thegrid spacing increases the resolution of the generated image. As shownin FIG. 8, inside target 118 occupies a single voxel 806A. Though thissimplifies the discussion, skilled artisans will recognize that inpractice a higher resolution will often be desired.

[0065]FIG. 9 is a flowchart that depicts step 410 in greater detailaccording to a preferred time domain interferometry technique forcalculating a value for each voxel 806 in imaging area 802. In step 902,a reflectogram is generated for each radar 202 in intrusion detectionsystem 100. FIG. 10 depicts four example reflectograms, 1002, 1004,1006, and 1008, corresponding to sensors 102A, 102B, 102C, and 102D,respectively. Skilled artisans will recognize that a reflectogramdescribes reflected energy as a function of range (i.e., distance fromthe transmitting antenna). For example, reflectogram 1002 describes thereflected energy measured at sensor 102A, whereas reflectogram 1004describes the reflected energy measured at sensor 102B. The x-axisrepresents range, while the y-axis represents reflected energy measuredas voltage.

[0066] In a preferred embodiment, each radar 202 generates areflectogram by sweeping through the ranges of interest, measuringreflected energy at discrete ranges. At each discrete range, radar 202transmits ultra wideband pulses 808 and then looks for reflected energyafter a time delay corresponding to the return time-of-flight. Furtherdetails regarding the operation of radar 202 are provided in U.S. Pat.Nos. 4,743,906, and 5,363,108, incorporated by reference above. Radar202 receives and, where multiple pulses are transmitted for eachdiscrete range step, accumulates reflected energy.

[0067] Those skilled in the art will recognize that more reflectedenergy will be measured per transmitted pulse for nearby targets, ascompared to those targets positioned farther away. Compensating for thiseffect allows for more efficient use of the radar's dynamic range. In apreferred embodiment, radar 202 transmits and receives an increasingnumber of pulses per discrete range step as the range is increased. Thereflected energy measured at longer ranges is therefore increased byreceiving and integrating a greater number of pulses. The ranges ofinterest are preferably divided into multiple “range windows,” where thesame number of pulses is transmitted for each discrete range within agiven window. Skilled artisans will recognize that this is only oneexample of how this compensation might be implemented.

[0068] Alternatively, the power of transmitted pulses can be variedaccording to range. In this embodiment, radar 202 increases the power oftransmitted pulses as the range gets longer. This alternativecompensation has a similar effect to varying the number of transmittedpulses, but will likely require more costly modifications to the basicradar 202 to implement. This, and other related concepts are describedin commonly owned, co-pending U.S. Pat. application Ser. No. ______(attorney docket 1659.0530000), filed the same date as the presentapplication, Jun. 14, 1999, entitled “System and Method for ImpulseRadio Power Control,” which is incorporated herein by reference.

[0069] Returning again to FIG. 9, in step 904 an image is formed byselectively combining data from the reflectograms generated in step 902.An image value is calculated for each voxel 806, where the image valueis indicative of the total amount of reflected energy measured over thatportion of imaging area 802. Processor 206 preferably calculates animage value for each voxel 806 by summing voltage values from thereflectogram associated with each sensor 102, where the voltage valuescorrespond to the return time-of-flight from the radar to the voxelbeing calculated. For example, referring to FIGS. 8 and 10, the imagevalue for voxel 806A is the sum of a voltage value from reflectograms1002, 1004, 1006, and 1008 corresponding to the return time-of-flight.As shown in reflectogram 1002, the voltage value at time t1 correspondsto the return time-of-flight from sensor 102A to voxel 806A, as shown inFIG. 8. Similarly, times t2, t3, and t4 correspond to the returntime-of-flight from sensors 102B, 102C, and 102D to voxel 806A, as shownin reflectograms 1004, 1006, and 1008. The sum of these four valuesforms the image value for voxel 806A.

[0070] In this manner the image value for each voxel 806 in image area802 is calculated as the sum of a voltage from each reflectogramcorresponding to the return time-of-flight.

[0071] Intrusion Detection

[0072] Returning to FIG. 3, in step 306, processor 206 determineswhether an alarm condition has been triggered indicating an intrusion.What is defined as an alarm condition depends upon the particularenvironment in which intrusion detection system is used. For example, ina home security environment, an alarm condition is triggered when amoving object approaches and penetrates a perimeter around the outsideof the house or some other predetermined exterior boundary.Alternatively, in a building security environment, movement in arestricted area within the building triggers an alarm condition. Thoseskilled in the art will recognize that alarm conditions will vary,depending upon the exact environment in which intrusion detection system200 is installed and the types of intrusion that are to be detected.

[0073] In a preferred embodiment, processor 206 uses the radar imagesgenerated in step 304 to detect motion and to track moving objects. Inmany instances, processor 206 need only detect movement in a given area.In the aforementioned building security environment, movement detectedin a restricted area triggers an alarm condition. Other alarm conditionsrequire additional processing to distinguish between different types ofmovement. For instance, movement in the vicinity of a window shouldtrigger an alarm condition if the object approached the window fromoutside 116, but not if the object approached from inside 114. Processor206 can distinguish between these two types of movement by trackingmoving objects over time.

[0074]FIG. 11 is a flowchart that depicts step 306 in detail accordingto a preferred embodiment. In step 1102, processor 206 updates a cluttermap. The clutter map represents stationary and other “don't care”objects within imaging area 802. For instance, a clutter map mightinclude stationary objects such as furniture and walls within abuilding. The clutter map can also include moving objects that shouldnot trigger an alarm condition, such as ceiling fans.

[0075] Those skilled in the art will recognize that the clutter map canbe determined in different ways. In one embodiment, the first radarimage generated by intrusion detection system 200 is defined as theclutter map. This approach is easy to implement, but is not very robust.For instance, if a piece of furniture within imaging area 802 is movedafter the clutter map is generated, it will thereafter appear as amoving object because it was not part of the clutter map. In thisembodiment, processor 206 sets the clutter map equal to the first radarimage generated in step 304, and does not change the clutter map basedon subsequent radar images.

[0076] In a preferred embodiment, however, the clutter map is updatedbased on subsequent radar images by low-pass filtering the current radarimage on a voxel by voxel basis, and adding the filtered image to thestored clutter map. In this way, the clutter map is slowly updated overtime so that stationary objects not present initially will beincorporated into the clutter map. For example, if sensors 102 transmitUWB pulses with a center frequency of 2 GHz, and if the 3 dB knee of thelowpass filter is 0.1 Hz, then anything moving at a rate faster than ¾inches in 10 seconds will not be passed through the lowpass filter tothe clutter map.

[0077] In step 1104, processor 206 subtracts the updated clutter mapfrom the current radar image. The resulting image represents objectswithin imaging area 802 that were not present in past radar images. FIG.12A depicts an example clutter map 1200 of building environment 100,including stationary objects such as doors 110, windows 108, interiorwalls 112 and exterior walls 106 (assume that everything shown in FIG.12A is within imaging area 802). FIG. 12B depicts a radar image 1202generated subsequent to clutter map 1200. As shown, inside target 118has entered the building. FIG. 12C depicts a differential map 1204calculated in step 1104 by subtracting clutter map 1200 from radar image1202. Differential map 1204 therefore represents objects that have movedwithin imaging area 802. The appearance of inside target 118 willtrigger an alarm condition for those intrusion detection systems thatare configured to detect movement in that particular area.

[0078] In step 1106, a track file is updated based on differential map1104 calculated in step 1104. The track file contains information onmoving objects being tracked within imaging area 802. For example, atrack file is a collection of historical information on identifiedobjects to allow determination of object motion parameters, such asposition, speed, velocity, and direction. In a preferred embodiment,objects that appear in differential map 1204 are compared against thoseobjects currently being tracked in the track file. Each object indifferential map 1204 is either associated with and used to update anexisting object in the track file, or is added to the track file as anew object to track.

[0079] One method of generating a track file is to map an area usingreflectogram data from several sensors, and then later, map the areaagain and subtract the first map data to derive a map of changesrelating to motion in the area. The largest peaks are then identified asobjects to be tracked and all energy within a radius (e.g., 1 foot) ofeach peak is considered part of the object. The object centroid is thenfound by determining the centroid of all of the “change” signal withinthe radius. This set of centroids is then compared with previouscentroids from the track file. The nearest previous object would beconsidered the same object of the purposes if determining object motion,velocity, direction. These parameters may be determined from the historyof the object centroid locations.

[0080] A track file may alternatively be maintained by determining anarea within some range (e.g., 1 foot) of a previous centroid locationfor an object, and then computing a new centroid based on this area tobe associated with the object. In this way, an object may beincrementally tracked across a room and objects may be determined asentering or exiting a door or widow.

[0081] Map threshold levels may be used to limit the number of objectsto a reasonable level. Objects may disappear, or be dropped from thetrack file, if the total energy drops below a disappearance thresholdfor a period of time. Likewise objects may be generated based on asingle peak threshold crossing, but may not achieve full “object” statusuntil it maintains threshold for a period of time.

[0082] Tracking the movement of objects within imaging area 802 allowsfor more sophisticated alarm conditions to be defined. For instance, inthe home security environment described above, an alarm condition mightbe triggered where outside target 120 approaches window 108, whereasinside target 118 approaching window 108 does not. Those skilled in theart will recognize the many ways that tracking could be used to definerobust alarm conditions in a variety of environments.

[0083] Calibration

[0084] Returning to FIG. 3, intrusion detection system 200 is calibratedin step 302 prior to generating a radar image in step 304 and detectingintrusion in step 306. The processing described above with respect tosteps 304 and 306 depends, in part, on having accurate knowledge ofwhere the sensors are located with respect to one another. Calibratingintrusion detection system 200 refers to determining these relativepositions.

[0085]FIG. 13 depicts a first alternative calibration system forintrusion detection system 200. A portable transmitter 1302 is movedalong a calibration path 1304 around the area to be protected. For theexample shown in FIG. 13, calibration path 1304 follows the outsidewalls 106 of the building. Those skilled in the art will recognize thatcalibration path 1304 will vary for different environments and alarmconditions. Portable transmitter 1302 transmits UWB pulses, such as aTM-UWB radar 202.

[0086] All of the sensors 102 lock their receivers to transmitter 1302and track its movement around calibration path 1304. As the sensors 102track transmitter 1302, datum marks are made periodically. This ispreferably accomplished by the operator pressing a button that modulatestransmitter 1302, sending a bit stream to each sensor 102 identifyingthe index number of the data point being sent. Alternatively, areal-time clock can be used to continually mark the data received by thesensors 102. In either case, after completion each sensor 102 sends thecalibration data to processor 206 to determine the position of thesensors 102 in relation to each other and calibration path 1304.

[0087] In a second alternative embodiment, in step 302, the calibrationis performed manually, by locating each sensor on a map, blueprint,survey, or by direct measurement. The calibration data is entered intoprocessor 206 by conventional means familiar to those skilled in theart.

[0088] In a third alternative embodiment, in step 302, each sensor 102locks on to UWB pulses transmitted by another sensor 102, one afteranother, until a range is determined between each pair of sensors 102.The sensors can be adapted to perform range finding as described incommonly owned, co-pending U.S. Pat. application Ser. No. 09/045,929,attorney docket no. 1659.0470000, filed Mar. 23, 1998, entitled “Systemand Method For Position Determination By Impulse Radio,” which isincorporated herein by reference. Another alternative embodiment foradapting the sensors to perform range finding is described in commonlyowned, co-pending U.S. Pat. application Ser. No. 09/083,993, attorneydocket no. 1659.0660000, filed May 26, 1998, entitled “System and MethodFor Distance Measurement by Inphase and Quadrature Signals In A RadioSystem,” which is also incorporated herein by reference. Each sensor 102sends the calibration data to processor 206 to determine the position ofthe sensors 102 in relation to each other.

[0089] Conclusion

[0090] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. Thus, the breadth and scopeof the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

[0091] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

1. An intrusion detection system comprising: a first time modulatedultra wideband (TM-UWB) radar, wherein said first TM-UWB radar isadapted to transmit UWB pulses and receive signal returns, and whereinsaid UWB pulses have a wavelength corresponding to the center frequencyof said UWB pulses; a second TM-UWB radar, spaced a distance from saidfirst TM-UWB radar, said distance being greater than ¾ of saidwavelength, wherein said second TM-UWB radar is adapted to transmitfurther UWB pulses, and wherein said further UWB pulses have saidwavelength; and a processor in communications with at least said firstTM-UWB radar, wherein said processor detects intrusion based on saidsignal returns.
 2. The system of claim 1 , wherein said first TM-UWBradar receives signal returns that correspond to said UWB pulsestransmitted by said first TM-UWB radar, and wherein said processordetects intrusion based on at least said signal returns that correspondto said UWB pulses transmitted by said first TM-UWB radar.
 3. The systemof claim 1 , wherein said first TM-UWB radar receives signal returnsthat correspond to said further UWB pulses transmitted by said secondTM-UWB radar, and wherein said processor detects intrusion based on atleast said signal returns that correspond to said further UWB pulsestransmitted by said second TM-UWB radar.
 4. The system of claim 1 ,wherein said first TM-UWB radar receives signal returns that correspondto said UWB pulses transmitted by said first TM-UWB radar and signalreturns that correspond to said further UWB pulses transmitted by saidsecond TM-UWB radar, and wherein said processor detects intrusion basedon said signal returns that correspond to said UWB pulses transmitted bysaid first TM-UWB radar and said signal returns that correspond to saidfurther UWB pulses transmitted by said second TM-UWB radar.
 5. Thesystem of claim 1 , wherein said processor detects intrusion bydetermining whether an alarm condition is met based on said signalreturns.
 6. The system of claim 1 , wherein said processor detectsintrusion by detecting movement of an object in a given area based onsaid signal returns.
 7. The system of claim 6 , wherein the objectcomprises one or more human beings positioned outside of a building andwherein said first and second TM-UWB radars are positioned inside thebuilding.
 8. The system of claim 6 , wherein the given area includes apredetermined boundary, and wherein said processor triggers an alarmwhen said processor detects penetration of the boundary by the object.9. The system of claim 8 , wherein the boundary comprises a first sideand a second side, and wherein said processor only triggers the alarmwhen said processor detects movement toward the first side of theboundary.
 10. The system of claim 8 , wherein the boundary is defined bya location of a window.
 11. The system of claim 6 , wherein the givenarea comprises a predetermined boundary, and wherein said processortriggers an alarm when said processor detects movement toward, andpenetration of, the boundary by the object.
 12. The system of claim 11 ,wherein the boundary includes a first side and a second side, andwherein said processor only triggers the alarm when said processordetects movement toward, and penetration of, the first side of theboundary.
 13. The system of claim 11 , wherein the boundary is definedby a location of a window.
 14. The system of claim 11 , wherein thefirst side of the boundary is outside a building, and wherein the secondside of the boundary is inside the building.
 15. The system of claim 1 ,wherein said processor generates an image based on said signal returnsand detects intrusion based on said image.
 16. The system of claim 15 ,wherein said processor generates a reflectogram based on said signalreturns and generates said image based on said reflectograms.
 17. Thesystem of claim 1 , wherein said processor generates an image based onsaid returns signals, subtracts said image from a clutter map to therebycreate a differential map, and detects intrusion based on saiddifferential map.
 18. The system of claim 17 , wherein said processortriggers an alarm when it detects intrusion, and wherein said cluttermap represents objects that should not cause said processor to triggerthe alarm.
 19. The system of claim 17 , wherein said processor updates atrack file based on said differential map, tracks movement in a givenarea based on said track file, and detects intrusion based on themovement.
 20. The system of claim 1 , wherein said first and secondTM-UWB radars are positioned along the perimeter of a building.
 21. Thesystem of claim 1 , further comprising: a first wireless link coupled tosaid processor; and a second wireless link coupled to said second TM-UWBradar, wherein said second wireless link communicates with said firstwireless link, wherein said second TM-UWB radar receives signal returnsthat correspond to UWB pulses transmitted by said second TM-UWB radar,and wherein said processor detects intrusion based on said signalreturns received by said first and second TM-UWB radars.
 22. The systemof claim 1 , wherein said first TM-UWB radar receives signal returnsthat correspond to UWB pulses transmitted by said second TM-UWB radar,and wherein said first and second TM-UWB radars are synchronized. 23.The system of claim 1 , wherein said second TM-UWB radar receives signalreturns that correspond to UWB pulses transmitted by said second TM-UWBradar, wherein said first TM-UWB radar receives signal returns thatcorrespond to UWB pulses transmitted by said second TM-UWB radar,wherein said first and second TM-UWB radars are synchronized, andwherein said processor detects intrusion based on said signal returnsreceived by said first and second TM-UWB radars.
 24. An intrusiondetection system comprising: a first time modulated ultra wideband(TM-UWB) radar, wherein said first TM-UWB radar is adapted to transmitUWB pulses in a given area and to receive signal returns, wherein saidUWB pulses have a wavelength corresponding to the center frequency ofsaid UWB pulses; a second TM-UWB radar, spaced a distance from saidfirst TM-UWB radar, said distance being greater than ¼ of saidwavelength, wherein said second TM-UWB radar is adapted to transmitfurther UWB pulses in said given area, wherein said further UWB pulseshave said wavelength; and a processor in communications with at leastsaid first TM-UWB radar, wherein said processor detects motion withinsaid given area based on said signal returns.
 25. The system of claim 24, wherein said first TM-UWB radar receives signal returns thatcorrespond to said UWB pulses transmitted by said first TM-UWB radar,and wherein said processor detects intrusion based on at least saidsignal returns that correspond to said UWB pulses transmitted by saidfirst TM-UWB radar.
 26. The system of claim 24 , wherein said firstTM-UWB radar receives signal returns that correspond to said further UWBpulses transmitted by said second TM-UWB radar, and wherein saidprocessor detects intrusion based on at least said signal returns thatcorrespond to said further UWB pulses transmitted by said second TM-UWBradar.
 27. The system of claim 24 , wherein said first TM-UWB radarreceives signal returns that correspond to said UWB pulses transmittedby said first TM-UWB radar and signal returns that correspond to saidfurther UWB pulses transmitted by said second TM-UWB radar, and whereinsaid processor detects intrusion based on said signal returns thatcorrespond to said UWB pulses transmitted by said first TM-UWB radar andsaid signal returns that correspond to said further UWB pulsestransmitted by said second TM-UWB radar.
 28. A method for detectingintrusion, comprising the steps of: a. transmitting ultra wideband (UWB)pulses from a first location, wherein said UWB pulses have a wavelengthcorresponding to the center frequency of said UWB pulses; b.transmitting further UWB pulses having said wavelength from a secondlocation, wherein said second location is spaced a distance from saidfirst location, said distance being greater than ¼ of said wavelength;c. receiving signal returns at at least said first location; and d.detecting intrusion based on said signal returns.
 29. The method ofclaim 28 , wherein said step d. comprises the steps of: i. generating animage based on said signal returns; and ii. detecting intrusion based onsaid image.
 30. The method of claim 28 , wherein said step d. comprisesthe steps of: i. generating reflectogram data based on said signalreturns; ii. generating an image based on said reflectogram data; andiii. detecting intrusion based on said image.
 31. The method of claim 30, wherein said step d.iii. comprises the steps of: (1) subtracting saidimage from a clutter map to thereby create a differential map; and (2)detecting intrusion based on said differential map.
 32. The method ofclaim 31 , wherein said step d.iii.(2) comprises the steps of: (a)updating a track file based on said differential map; (b) trackingmovement of an object based on said track file; and (c) triggering analarm based on said movement.
 33. The method of claim 28 , furthercomprising the step of determining relative positions of said firstlocation and said second location with respect to one another prior tosaid step d..
 34. The method of claim 33 , wherein said step ofdetermining relative positions comprises the steps of: i. receivingadditional UWB pulses, at said first and second locations; ii.generating calibration data based on said additional UWB pulses; andiii. determining said relative positions based on said calibration data.35. The method of claim 34 , wherein said additional UWB pulses aregenerated by a transmitter moving along a calibration path.
 36. Themethod of claim 28 , wherein said step d. comprises the step of: i.determining whether an alarm condition has been met based on said signalreturns.
 37. The method of claim 28 , wherein step c. comprises the stepof receiving signal returns at said first location and said secondlocation, and wherein step d. comprises the step of detecting intrusionbased on said signal returns received at said first location and saidsecond location.